1. Field of the Invention
The present invention relates to a zoom lens system composed of a small number of lenses having very light weight and extremely reduced manufacturing cost and, more particularly, to an ultimately compact standard zoom lens system.
2. Description of Related Art
Recently, a standard lens attached to a single lens reflex camera has been replaced with a so-called standard zoom lens, and the trend has taken a firm hold on the customer as a lens in common use. Accordingly, it is indispensable condition that the standard zoom lens like this, which is always attached to a camera body while being moved around, is compact and lightweight, has sufficient imaging-forming performance, and is also inexpensive. In order to satisfy the condition, it is the most suitable for the standard zoom lens to be a two-unit zoom lens system consisting of a negative lens unit and a positive lens unit. Therefore, regarding this type of a zoom lens system, various lens arrangements have been proposed. Above all, a compact zoom lens with relatively fewer lens components is disclosed in Japanese Patent Application Laid-Open No. 56-43619. Japanese Patent Application Laid-Open No. 57-20713 discloses a zoom lens improved weight reducing and cost lowering by introducing a plastic lens element with an aspherical surface. Moreover, Japanese Patent Application Laid-Open No. 1-239516 discloses an example of a zoom lens with increased zoom ratio.
Although the zoom lens system disclosed in Japanese Patent Application Laid-Open No. 56-43619 has a relatively small number of lens elements, the zoom ratio about 1.9 is not a satisfactory specification as a current standard zoom lens. Moreover, the power arrangement of each lens unit is relatively loose, so that it has a zoom trajectory that the total lens length in the wide-angle state, which gives the shortest focal length, becomes excessively long. Accordingly, each lens element in the first lens unit having a negative refractive power is very large, so that it causes problems regarding the overall length and weight. Furthermore, glass materials used by respective lens elements in the first lens unit, which has the largest glass volume, are relatively expensive, so that it has been needed that the manufacturing cost is further lowered.
In the zoom lens system disclosed in Japanese Patent Application Laid-Open No. 57-20713 having a plastic lens element with an aspherical surface, the zoom ratio of 1.9 is not a satisfactory specification as a current standard zoom lens. Moreover, since the surface variation of a plastic lens with an aspherical surface, which is used in the zoom lens system, has been greatly affected by the temperature variation or the moisture absorption, there has been a problem on quality assurance as a photographic objective lens. Furthermore, since the manufacturing cost for processing a high precision plastic lens having processing of an aspherical surface liable to become higher than that of a polished glass lens using a small, cheep glass material, there is a problem of lowering the cost. Moreover, since the power arrangement of each lens unit is relatively loose, it has a zoom trajectory that the total lens length in the wide-angle state becomes excessively long. Accordingly, each lens element in the first lens unit having a negative refractive power is very large, so that it causes problems regarding the total lens length and weight. Furthermore, glass materials used by respective lens elements in the first lens unit, which has the largest glass volume, are relatively expensive, so that the manufacturing cost has been needed to be further lowered. Moreover, the number of lens elements comprising the second lens unit is large, so that further lowering the cost has been needed.
In the zoom lens system disclosed in Japanese Patent Application Laid-Open No. 1-239516, although the zoom ratio of the lens system has a sufficient value of 2.3, it is disadvantageous for the cost that the lens system uses eight lens elements, which is rather large number, and a lens with an aspherical surface. Furthermore, glass materials used by respective lens elements in the first lens unit, which has the largest glass volume, are relatively expensive and heavy, so that the manufacturing cost and the weight has been needed to be further lowered. Moreover, the number of lens elements comprising the second lens unit is large, so that further lowering the cost has been needed.
The present invention is made in view of the aforementioned problems and has an object to provide a zoom lens system being extremely reduced size such as a standard single focal length lens, having very few number of lens elements, being ultimately reduced cost, having a zoom ratio about 2.2, and also having high optical performance without using any aspherical surface.
According to one aspect of the present invention, a zoom lens system includes, in order from an object side, a first lens group having a negative refractive power and a second lens group having a positive refractive power. A zooming is performed by varying an air space between the first lens group and the second lens group. The first lens group comprises, in order from the object side, a negative lens group consisting of one or two negative lens elements and a positive lens element having a convex surface facing to the object side. All lens surfaces composing the first lens group are spherical or plane. The following conditional expressions (1) and (2) are satisfied:
1.4xe2x89xa6Nnxe2x89xa61.621xe2x80x83xe2x80x83(1) 
0.7xe2x89xa6|f1|/(fwxc2x7ft)xc2xdxe2x89xa61.15xe2x80x83xe2x80x83(2) 
where Nn denotes an average refractive index of the negative lens group within the first lens group for the d-line, f1 denotes the focal length of the first lens group, fw denotes the focal length of the zoom lens system in a wide-angle state, which gives the shortest focal length, and ft denotes the focal length of the zoom lens system in a telephoto state, which gives the longest focal length.
In one preferred embodiment of the present invention, the first lens group consists of, in order from the object side, a negative lens group consisting of one or two negative lens elements and a positive lens element having a convex surface facing to the object side.
In one preferred embodiment of the present invention, the following conditional expression (3) is satisfied:
1.9xe2x89xa6SGxe2x89xa64xe2x80x83xe2x80x83(3) 
where SG denotes an average value of the specific gravity of each glass material composing the negative lens group within the first lens group consisting of one or two negative lens elements.
In one preferred embodiment of the present invention, the following conditional expression (4) is satisfied:
0.5 less than f2/fw less than 1.1xe2x80x83xe2x80x83(4) 
where f2 denotes the focal length of the second lens group.
In one preferred embodiment of the present invention, the following conditional expression (5) is satisfied:
0.693 less than Nnxc2x7fw/ft less than 0.753xe2x80x83xe2x80x83(5). 
According to another aspect of the present invention, a zoom lens system includes, in order from an object side, a first lens group having a negative refractive power and a second lens group having a positive refractive power. A zooming is performed by varying an air space between the first lens group and the second lens group. The first lens group comprises, in order from the object side, a negative lens group having one or two negative lens elements and a positive lens element consisting of a convex surface facing to the object side. The second lens group comprises, in order from the object side, two positive lens elements, a negative lens element, and a positive lens element. The following conditional expression (1) is satisfied;
1.4xe2x89xa6Nnxe2x89xa61.621xe2x80x83xe2x80x83(1) 
where Nn denotes an average refractive index of the negative lens group within the first lens group for the d-line.
According to another aspect of the present invention, a zoom lens system includes, in order from an object side, a first lens group having a negative refractive power and a second lens group having a positive refractive power. A zooming is performed by varying an air space between the first lens group and the second lens group. The first lens group comprises, in order from the object side, a negative lens group consisting of one or two negative lens elements and a positive lens element having a convex surface facing to the object side. The second lens group comprises, in order from the object side, two positive lens elements, a negative lens element, and a positive lens element. The following conditional expression (3) is satisfied:
1.9xe2x89xa6SGxe2x89xa64xe2x80x83xe2x80x83(3) 
where SG denotes an average value of the specific gravity of each glass material composing the negative lens group within the first lens group consisting of one or two negative lens elements.
According to another aspect of the present invention, a zoom lens system includes, in order from an object side, a first lens group having a negative refractive power and a second lens group having a positive refractive power. A zooming is performed by varying an air space between the first lens group and the second lens group. The first lens group comprises, in order from the object side, a negative lens group consisting of one or two negative lens elements and a positive lens element having a convex surface facing to the object side wherein all lens surfaces composing the first lens group are spherical or plane. The following conditional expressions (2) is satisfied:
0.7xe2x89xa6|f1|/(fwxc2x7ft)xc2xdxe2x89xa61.15xe2x80x83xe2x80x83(2) 
where f1 denotes the focal length of the first lens group, fw denotes the focal length of the zoom lens system in a wide-angle state, which gives the shortest focal length, and ft denotes the focal length of the zoom lens system in a telephoto state, which gives the longest focal length.